1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a transflective liquid crystal display device and a fabricating method thereof.
2. Discussion of the Related Art
Flat panel display (FPD) devices having portability and low power consumption have been a subject of increasing research in the present information age. Generally, FPD devices may be classified according to their ability for self-emission, and may include emissive display devices and non-emissive display devices. The emissive display devices display images by taking advantage of their ability to self-emit light, and the non-emissive display devices require a light source since they do not emit light by themselves. For example, plasma display panel (PDP) devices, field emission display (FED) devices, and electroluminescent display (ELD) devices are commonly used as emissive display devices. Liquid crystal display (LCD) devices may be categorized as non-emissive display devices and are commonly used in notebook and desktop computers because of their high resolution, capability of displaying colored images, and high quality image display.
In general, an LCD device includes two substrates disposed such that respective electrodes of the two substrates face into each other. A liquid crystal layer is interposed between the respective electrodes. When a voltage is applied to the electrodes, an electric field is generated and the electric field modulates the light transmittance of the liquid crystal layer by reorienting liquid crystal molecules, thereby displaying images in the LCD device.
Since the LCD device is a non-emissive-type display device, an additional light source is required. Thus, a backlight unit is disposed under an LCD panel, wherein the LCD device displays images using light produced by the backlight unit. The respective electrodes of the two substrates of the LCD device may be formed of a transparent conductive material and the two substrates of an LCD device may be formed of a transparent material. The above-mentioned LCD device is referred to as a transmissive type LCD device. Even though the transmissive type LCD device displays bright images under dark surroundings due to an artificial light source such as a backlight unit, power consumption of the LCD device increases due to the backlight unit.
In order to improve above disadvantages of the transmissive type LCD device, a reflective type LCD device has been suggested. Since the reflective type LCD device uses the ambient artificial or natural light instead of light from the backlight unit, power consumption of the reflective type LCD device is smaller than that of the transmissive type LCD device. In the reflective LCD device, a reflective layer of a metallic material having a high reflectance is formed on a first substrate and a common electrode of a transparent conductive material is formed on a second substrate. Even though the low power consumption of the reflective type LCD device is improved, the reflective type LCD device may not be used where the ambient light is weak or does not exist due to low brightness.
In order to overcome such problems, a transflective LCD device has been researched and developed. The transflective LCD device can be switched from a transmissive mode using transmission of light to a reflective mode using reflection of light according to the user's selection. Accordingly, disadvantages of the transmissive type LCD device and the reflective type LCD device such as high power consumption and low brightness under dark surroundings are improved.
FIG. 1 is a schematic cross-sectional view of a transflective liquid crystal display device having a single cell gap according to the related art.
In FIG. 1, a gate electrode 6 is formed on a first substrate 2 and a gate insulating layer 10 is formed on the gate electrode 6. Even though not shown in FIG. 1, a gate line connected to the gate electrode 6 is formed under the gate insulating layer 10. An active layer 13 and an ohmic contact layer 16a and 16b are sequentially formed on the gate insulating layer 10 over the gate electrode 6. Source and drain electrodes 23 and 26 are formed on the ohmic contact layer 16a and 16b. The source and drain electrodes 23 and 26 constitute a thin film transistor (TFT) “Tr” with the gate electrode.
A data line 20 of the same material as the source and drain electrodes 23 and 26 is formed on the gate insulating layer 10. Even though not shown in FIG. 1, the data line 20 is connected to the source electrode 23. The data line 20 crosses the gate line (not shown) to define a pixel region “SP.” A first passivation layer 30 of an organic material having a low dielectric constant is formed on the TFT “Tr.” A reflective layer 40 of a metallic material having a high reflectance is formed on the first passivation layer 30 in a reflective portion “RA.” The reflective layer 40 has an open portion 40a corresponding to a transmissive portion “TA.” A second passivation layer 45 of an inorganic material is formed on the reflective layer 40. A pixel electrode 50 connected to the drain electrode 26 through a contact hole 55 is formed on the second passivation layer 45 in each pixel region “SP.”
A black matrix 75 is formed on a second substrate 71 and a color filter layer 80 including red, green and blue sub-color filters 80a, 80b and 80c is formed on the black matrix 75. An overcoat layer 85 and a common electrode 90 of a transparent conductive material are sequentially formed on the color filter layer 80. One sub-color filter 80a, 80b and 80c corresponds to one pixel electrode 50. The black matrix 75 overlaps edge of the pixel electrode 50 and corresponds to the data line 20.
A liquid crystal layer 60 is formed between the pixel electrode 50 and the common electrode 90. When a voltage is applied to the pixel electrode 50 and the common electrode 90, arrangement state of liquid crystal molecules in the liquid crystal layer 60 changes according to an electric field generated between the pixel electrode 50 and the common electrode 90. Even though not shown in FIG. 1, respective orientation films are formed on the pixel electrode 50 and the common electrode 90 to determine an initial arrangement state of the liquid crystal molecules. Respective retardation films 97 and 95 are formed outsides of the first and second substrates 2 and 71 to adjust phase of light.
In the transflective LCD device of FIG. 1, a first cell gap “d1” of the reflective portion “RA” is similar to a second cell gap “d2” of the transmissive portion “TA.” Accordingly, cell efficiency between the reflective portion “RA” and transmissive portion “TA” is not optimized. These cause problems such as reduction of transmittance and brightness. To overcome above problems, a transflective LCD device having a dual cell gap is suggested.
FIG. 2 is a schematic cross-sectional view of a transflective liquid crystal display device having a dual cell gap according to the related art. Illustration for the same portions as the transflective LCD device of FIG. 1 will be omitted.
In FIG. 2, a first passivation layer 30 has an open portion 30a corresponding to a transmissive portion “TA.” Accordingly, a first cell gap “d3” of the reflective portion “RA” is a half of a second cell gap “d4” of the transmissive portion “TA.” A liquid crystal layer 60 includes liquid crystal molecules of an electrically controlled birefringence (ECB) mode. When liquid crystal molecules of an ECB mode are used for an LCD device, transmittance curve of the LCD device has periodic peaks every multiple of a unit cell gap. Accordingly, light efficiency in the reflective portion “RA” is similar to that in the transmissive portion “TA” and cell efficiency of the reflective portion “RA” and the transmissive portion “TA” is maximized.
The reflective LCD devices of FIGS. 1 and 2, however, have problems such that color property is reduced in the transmissive portion. While light passes through the color filter layer twice, i.e., before and after reflection in the reflective portion, light from the backlight unit passes the color filter layer once in the transmissive portion. Accordingly, light emitted from the transmissive portion is different from that from the reflective portion in color property. Moreover, reflection efficiency is not maximized due to flatness of the reflective layer. Recently, to overcome the above problems, a transflective LCD device has been suggested that incorporates a color filter layer with a through hole and an uneven reflective layer.
FIG. 3 is a schematic cross-sectional view of a transflective liquid crystal display device having a dual cell gap, a color filter layer including a through hole and a reflective layer having unevenness according to the related art. Illustration for the same portions as the transflective LCD devices of FIGS. 1 and 2 will be omitted.
In FIG. 3, a color filter layer 80 including red, green and blue sub-color filters 80a, 80b and 80c has a through hole “TH” to adjust color property and brightness. The through hole “TH” does not include color resin for the color filter layer 80, and color property and brightness may be adjusted by varying number and size of the through hole “TH.” Since the through hole “TH” corresponds to a reflective portion “RA,” color property and brightness of light emitted from the reflective portion “RA” becomes similar to those of light emitted from a transmissive portion “TA.” Moreover, since a reflective layer 41 includes unevenness, reflection efficiency is improved.
First and second passivation layers 30 and 31 are sequentially formed on a thin film transistor (TFT) “Tr.” The first passivation layer 30 has a protrusion 32 for unevenness of the reflective layer 41 and an open portion 30a for a dual cell gap such that a first cell gap “d3” in the transmissive portion “TA” is a double of a second cell gap “d4” in the reflective portion “RA.” The protrusion 32 and the open portion 30a may be formed through one etching process. However, it is hard to obtain an optimum process condition for the protrusion 32 and the open portion 30a simultaneously. Moreover, the optimum process condition for the protrusion 32 and the open portion 30a may be changed according to the number and the size of the through hole “TH” of the color filter layer 80. Accordingly, design and fabricating process for the transflective LCD device are not stable, and inferiority may occur during the fabricating process.